The present invention relates to a process for removing 1,4-naphthoquinone from phthalic anhydride.
It is known from Ullmann, volume 13, pages 720- 721, that phthalic anhydride which has been obtained by gas phase oxidation of naphthalene can contain small quantities of 1,4-naphthoquinone. Further, it is known from this publication that 1,4-naphthoquinone can be removed by subjecting the crude phthalic anhydride to a heat treatment with concentrated sulphuric acid. After the heat treatment, the excess sulphuric acid is neutralised by adding chalk. Distillation then gives a phthalic anhydride which has been freed from 1,4-naphthoquinone. The disadvantage of this process is the corrosion occasioned by the presence of sulphuric acid and the formation of solid residues during distillation, occasioned by the addition of chalk, so that this process can for practical purposes only be carried out discontinuously.